This invention is generally directed to processes for the preparation of particles, and more specifically to processes for the preparation of small sized polymeric particles, for example, in one embodiment with an average diameter of from about 0.1 micron to about 40 microns. More specifically, the present invention is directed to processes for the preparation of particles by photochemical suspension polymerization methods. One embodiment of the present invention relates to a process for the preparation of particles by suspension polymerization and exposure to light, such as UV light, to accomplish stabilization thereof. With the processes of the present invention, there is enabled a simple economical one step method for obtaining particles, including toners, and wherein suspension failure is avoided or minimized. Also, with the processes of the present invention in some embodiments undesirable agglomeration or coalescence is eliminated or substantially avoided, and there can be obtained particles of small average diameters, for example, of from 0.1 to about 40 microns. As indicated herein, the particles obtained with the process of the present invention can, for example, be selected as toner polymer resins for toner and developer compositions, and these particles may also be selected as resin binders for liquid toners. As compared to known processes such as suspension polymerization and semisuspension polymerization, the processes of the present invention hves the advantages of permitting the formation of particles in a one step process. In semisuspension polymerization processes at least two process steps are needed. Furthermore, about 0.1 to about 40 microns average diameter particles cannot, it is believed, be effectively prepared by conventional suspension polymerization processes. Another advantage of the processes of the present invention resides in accomplishing the photostabilization both in a batch process and an economical continuous process.
Conventionally, the formation of small polymeric particles for use in dry toners, liquid toners and developers, nonimaging applications such as chromatographic supports and medical applications has been generally accomplished by emulsion polymerization, dispersion polymerization, suspension polymerization and semisuspension polymerization processes. Emulsion polymerization can be selected to prepare submicron particles and can yield larger particles only upon being submitted to a swelling process or multi-step swelling processes. Emulsion polymerizations are initiated with water soluble initiators and the solubility of the starting monomer in the water phase is an important factor since it influences the process kinetics. Also of interest is U.S. Pat. No. 4,486,559, which discloses the incorporation of a prepolymer into a monomer toner mix followed by emulsion polymerization; and U.S. Pat. Nos. 4,680,200 and 4,702,988, which illustrate emulsion polymerization.
Dispersion polymerizations are usually performed in an organic medium wherein all the monomers are soluble in the medium prior to the polymerization and insoluble after the polymerization. Solvent recovery in this process is very costly. Moreover, dispersion polymerization is rarely accomplished in a water medium. In dispersion polymerization, the stabilizer permits the formation of a protective layer on the surface of each particle thereby improving stability, however, this process does not, for example, usually effectively permit the incorporation of solid additives within the particles.
Suspension polymerization of monomer for the formation of polymer particles generally in a size range of about 200 microns and higher is known. The main advantage of suspension polymerization is that the product may easily be recovered, therefore, such a process is considered economical. However, it is very difficult by suspension polymerization to prepare very small particles as the monomer droplets tend to coalesce during the polymerization process, especially in the initial stage of polymerization where the droplets are very sticky. For example, there is disclosed in U.S. Pat. No. 3,243,419 a method of suspension polymerization wherein a suspending agent is generated during the suspension polymerization to aid in the coalescence of the particles. Also disclosed in U.S. Pat. No. 4,071,670 is a method of suspension polymerization wherein the monomer initiator mixture is dispersed in water containing stabilizer by a high shear homogenizer, followed by polymerization of suspended monomer droplets. Since the particles formed by suspension polymerization are generally larger than 50 microns and as large as about 200 microns in size, they are not as desirable as carrier powder coatings or as toner polymer resins.
There was recited in a patentability search report the following prior art, all U.S. Pat. Nos. 4,804,610 relating to processes for the preparation of toner by a suspension method with a variety of monomers, such as styrene, acrylic acid ester, or methacrylic acid ester, and polymerization initiator azo compounds; 4,626,489, a continuation-in-part of 4,601,968, relating to processes for the preparation of toners by suspension polymerization of a monomer, a polymerization initiator, and a colorant; and 4,659,641 which discloses a suspension polymerization method for the preparation of toners, and wherein surfactants such as sodium alkylnaphthalene sulfonate are selected.
Illustrated in copending applications U.S. Ser. No. 339,539/89, and U.S. Pat. No. 3,043,404, the disclosures of which are totally incorporated herein by reference, are semisuspension polymerizations for the preparation of particles, including submicron particles, and toners thereof. More specifically, there is illustrated in this copending application and patent (1) a process for the preparation of toner particles which comprises mixing at least one resin monomer with a polymerization initiator, a crosslinking component and a chain transfer component; effecting bulk polymerization until partial polymerization has been accomplished; mixing with the aforementioned partially polymerized monomer product pigment or dye particles, thereby forming an organic phase; dispersing the organic phase in water containing a stabilizing component whereby there is obtained a suspension of toner particles in water; and polymerizing the toner suspension; (2) a process for the preparation of toner particles which comprises mixing at least one resin monomer with a polymerization initiator, a crosslinking component and a chain transfer component; effecting bulk polymerization until from about 10 to about 50 weight percent of the monomer has polymerized; mixing with the aforementioned partially polymerized monomer product pigment or dye particles until an organic phase is formed; dispersing the organic phase in water containing a stabilizing component whereby there is obtained a suspension of toner particles in water; and polymerizing the toner suspension; and (3) a process for the preparation of polymeric particles which comprises mixing at least one monomer with a polymerization initiator, a crosslinking component and a chain transfer component; effecting bulk polymerization until from about 10 to about 50 weight percent of the monomer has been polymerized; dispersing the aforementioned partially polymerized product in water containing a stabilizing component to obtain a suspension of particles with an average diameter of from about 0.1 to about 5 microns in water; and polymerizing the resulting suspension. Semisuspension polymerization processes, such as those described in the aforementioned copending applications, are considered multi-step particle formation processes suitable for the preparation of particles with an average diameter of from, for example, about 0.1 to about 25 microns which preferably can be selected as powder coatings, toners and toner additives.
Other methods selected for the formation of small polymeric particles are also known wherein grinding or attrition, especially fluid energy milling, of large particles to the size desired for powder coating, that is for example from about 0.1 to about 5 microns, is often not desirable from, for example, an economic and functional viewpoint. Powder coatings with submicron particles are desirable as it allows complete coating of the carrier core, such as carrier beads, with a thinner film. Also, particles with average particle diameters of less than 40 microns are very difficult to obtain by grinding or attrition. Further, processes such as spray drying of polymers suspended in solvent can result in polymer particles with a wide size range as well as trapping of the solvent selected which interferes with the use of the particles obtained as for coatings. Moreover, solvent recovery processes can be costly.
There is disclosed in U.S. Pat. No. 3,505,434 a process wherein particles which can be used, for example, for fluidized bed powder coating are prepared by dispersing nonspherical polymer particles in a liquid which is heated to above the polymer melting point and stirred causing the polymer particles to become substantially spherical. The particles are then cooled below their melting point and recovered. However, this process does not, for example, readily enable particles with an average particle diameter size of below 50 microns since the original particles must be prepared by conventional grinding or attrition processes. This process is also limited by utilization of particles wherein the melting point of the particles is lower than the boiling point of the medium.
There is disclosed in U.S. Pat. No. 4,602,970 and European Patent 183890-B a process by which photochemical energy in combination with thermally initiated polymerization is selected to stabilize emulsions. The stabilization is accomplished, for example, by a surfactant, such as sorbitan monoleate, which contains a polymerizable double bond. With the process of the present invention, polymeric or low molecular weight surfactants containing reactive double bonds selected to stabilize the submicron suspensions can be avoided if desired. Although it is not desired to be limited by theory, stabilization for the processes of the present invention is, it is believed, the result of the photochemical grafting of surfactants with oil soluble monomers to form a protective layer at the surface of the nonpolymerized particles. These particles can then be polymerized thermally without agglomeration problems or coalescence. Also disclosed in U.S. Pat. No. 4,248,685 are polymerization processes involving both photochemical and thermal free radical initiation to form submicron emulsions, and wherein the initiators selected are dispersed in the water phase and not in the monomer phase. There is thus a need for polymerization processes which will allow the formation of from about 0.1 to about 40 average diameter micron particles in a water medium wherein polymerization initiators are not present in the water medium and are present and soluble in the monomeric medium.
There have been proposed processes for obtaining toner compositions with certain particle size distributions in a reactor. These processes include dispersion polymerization, suspension polymerization and emulsion polymerization as illustrated herein, and the like. Disclosed in U.S. Pat. No. 4,486,559, the disclosure of which is totally incorporated herein by reference, is the preparation of a toner composition by the incorporation of a prepolymer into a monomer/pigment mixture, followed by emulsion polymerization, see for example columns 4, 5 and 8 of this patent. Also, methods of preparing toner, including suspension/dispersion polymerization, are detailed in columns 1 and 2 of this patent. In these processes, the pigment and additives, such as charge control components, are added to a monomer or comonomers prior to polymerization. Particle formation is achieved by the dispersion of the pigmented monomer or comonomers in a continuous phase such as water, and the droplets of pigmented monomers are then polymerized to form toner particles. One advantage of these processes is the elimination of fusion mixing (Banbury/extruder) and pulverization classification processing. Nevertheless, it is usually difficult with these processes to accomplish polymerization of pigmented monomer droplets in an average size diameter of from about 5 to about 25 microns with a narrow particle diameter distribution of, for example, 1.3. Also, suspension failure is common with these processes especially when the monomer droplet diameter is less than 50 microns and as polymerization proceeds in the sticky region, which generally occurs at monomer conversions of from 10 to 50 percent. Moreover, the prior art grinding processes for the preparation of particles is usually not as economical as desired. Jetting or other mechanical grinding processes are costly since they are energy extensive processes and also, they produce materials with a wide particle size distribution which need classification, therefore resulting in losses of materials and lower yields. These, and other disadvantages may be avoided or minimized when the processes of the present invention are selected for the preparation of particles and toner compositions.
There thus remains a need for a simple, direct, economical process of obtaining small polymeric particles, and more specifically toner polymeric particles with an average size diameter of from about 0.1 to about 40 microns. Further, there is a need for suspension polymerization processes for obtaining small dry toner polymeric particles of, for example, from about 0.1 to about 40 microns in average size diameter as determined by a scanning electron microscope. Additionally, there is a need for improved suspension polymerization processes that permit dry polymeric particles that can be selected as toners, as carrier powder coatings, and as toner additives for photoreceptor cleaning. There is also a need for a process which allows in situ stabilization of submicron monomeric particles dispersed in a water medium thus minimizing the amount of stabilizer or surfactant needed for stabilizing the particles prior to further treatment such as heating and polymerization. Also, there is a need for a direct process for the preparation of toner polymer particles with an average diameter of from about 0.1 to about 40 microns by a polymerization mechanism in which the initiators are only present in the monomer phase.